This type of decelerator is well known, particularly in the field of industrial vehicles equipped with a combustion engine. Taking into account their high inertia, in addition to their own braking system, these vehicles require a decelerator device housed in the exhaust gas circuit, preferably between the engine and the silencer, and generally mounted on the outlet side of the turbocompressor fed by the exhaust head. It is usually controlled by a pedal activated by the driver's left foot to prevent the latter from disengaging the clutch at the same time. This decelerator device makes it possible to generate a back pressure in the exhaust circuit. This back pressure is high or low depending on the position of the shutter and its degree of reopening. It has the effect of slowing down the engine and therefore provides additional braking of the vehicle. The higher the back pressure is, the more efficient the deceleration is. However, this back pressure has to be limited to the maximum set-point pressure allowable by the exhaust head, in order to prevent the reopening of the inlet valves. This back pressure of course depends on the exhaust gas pressure exerted on the shutter which is an asymptotic function of the engine speed. Consequently, there is a level of back pressure and therefore a deceleration level which corresponds to each engine speed. What is currently being sought is to make this function constant and at most equal to the maximum allowable set-point pressure whatever the engine speed and right from the low speeds, with the aim of optimizing the deceleration.
One of the techniques proposed is described in the publication EP-A-536 284 in which an exhaust modulator device comprises two leak holes closed by spring blades with different rigidities, in a shutter. When a first level of back pressure is reached, a first spring blade opens a first leak hole and releases part of the exhaust gas. A second leak hole thus increases the exhaust gas flow rate. This device makes it possible to limit the back pressure but generates deceleration stages. It is therefore not optimum. Furthermore, the spring blades are subject to temperature constraints and to the corrosion due to the exhaust gas. As a result, the way they operate deteriorates in time, thus affecting their performance when opening and closing. Indeed, under the heat of the exhaust gas, the spring blades become less rigid and the holes will always stay open, thus reducing the back pressure and the deceleration efficiency. What is more, as the modulation system is incorporated into the shutter, it is impossible to offer it as part of the after-sales service to equip decelerator devices already in service.
Another technique is described in the publication FR-A-2 481 367 which provides for a device to balance the back pressure, which is mounted at the end of the cylinder rod and coupled to the shutter's lever by means of a ball joint. This device is therefore located outside and protrudes. It is consequently subject to the external attacks in the engine environment, i.e. splashes of water, mud, salt, oil, etc. What is more, it represents a by no means insignificant mass at the end of the rod, which causes premature wear and tear on the cylinder and a danger of the rod breaking but also generating a considerable increase in this rod's inertia, and therefore in the decelerator's opening and closing times. This device requires a part which is specifically adapted on the cylinder's rod, the latter only being guided in translation over a very small span which is able to cause a risk of blocking.
The publication U.S. Pat. No. 4,669,585 describes a technique which is similar to the one above but adapted to a slide valve, whose overall height is much greater than that of a butterfly valve. The device for balancing the back pressure comprises an opening in the shutter sealed by a plate coupled to the end of a cylinder rod which can slide in the piston against the action of a spring when the back pressure reaches a certain threshold. This plate only offers the exhaust gas a small contact surface, which reduces the decelerator's efficiency. Furthermore, this device is subject to direct attacks from the exhaust gas and can, as a result, become jammed or be damaged prematurely, which reduces the device's reliability. This device is also complex and expensive to implement. It is also subject to parasitic forces, the back pressure acting perpendicular to the movement of said plate which further reduces the device's efficiency and reliability.